Current treatment of severe dental pain includes root canal therapy or tooth extraction. Obtaining adequate anesthesia for these invasive procedures is often difficult. Failure to obtain complete, or profound anesthesia is observed in as many as 18% of patients undergoing root canal treatment. The result is that a significant number of patients must withstand intense pain for the duration of the procedure. In addition to several long-term deleterious consequences of such painful experiences, the fact that the inflamed pulp is more difficult to anesthetize suggests that more local anesthetic (LA) will be employed in an effort to obtain a sufficient level of anesthesia, thereby increasing the likelihood of systemic and/or local toxicity. Whereas a number of explanations for LA failure have been proposed, none are able to account for the nature and extent of LA failure observed in the presence of pulpitis. In the present application, we propose to test a novel hypothesis concerning the basis of LA failure in the presence of pulpal inflammation: that LA failure is due to a change in voltage-gated Na+ channel (VGSC) subunits along the axons of pulpal afferents. Importantly, these changes may also play a significant role in the pain associated with inflammation within the oral cavity. We proposed to test this hypothesis in experiments described under 3 Specific Aims in which we will: determine whether the pulpal inflammation-induced decrease in LA sensitivity in the rat reflects a change in the biophysical properties, relative density and/or pattern of expression of VGSC subunits in pulpal afferents (Specific Aim 1); determine whether there is an association between pulpal inflammation, a decrease in LA sensitivity and changes in the relative density of VGSC subunits in human teeth (Specific Aim 2); and evaluate the basis for a causal link between pulpal inflammation, changes in VGSC subunit expression and changes in LA sensitivity in the rat (Specific Aim 3). Importantly, results from these experiments will not only enable us to determine whether changes observed in the rat may contribute to LA failure in humans, but will enable us to establish a causal link between changes in subunit expression and LA failure. Results from these experiments may not only enable identification of ways to make root canal treatment a more tolerable procedure, but may yield novel therapeutic interventions for the treatment of pain associated with both chronic inflammation and nerve injury.